Recently, display apparatus such as projectors, television receivers, and computer display units which employ optical devices such as liquid crystal panels or the like are in widespread use. In a display apparatuses which employs a liquid crystal panel or the like, light emitted from a light source such as a metal halide lamp, a halogen lamp, or the like is applied to a liquid crystal panel having color filters (R, G, B), and the liquid crystal panel displays a colored video image as its output light. The output light from the liquid crystal panel is projected onto a screen by a projection lens.
Light radiated from an ordinary light source has two planes of polarization that are generally referred to as a P-polarized component (hereinafter referred to as a P wave) and an S-polarized component (hereinafter referred to as an S wave). The display apparatus has polarizing means positioned such that light emitted from the light source is applied to the polarizing means before being applied to the liquid crystal panel. The polarizing means applies light having a plane of polarization which is either the P wave or the S wave depending on a polarizer disposed in front of the liquid crystal panel.
If rays of randomly polarized light are applied to polarizing beam splitters (hereinafter referred to as a PBS) disposed in prisms, for example, at a given angle, then a P wave passes through the PBSs and an S wave is reflected by the PBSs. Both the P and S waves are refracted by end faces of the prisms into parallel rays of light, and only the S wave is converted into a P wave by being transmitted through a .lambda./2 plate. Alternatively, the S wave is converted into a P wave by being refracted by end faces of the prisms so as to be parallel to the direction of travel of the P wave that has passed through the PBSs, or reflected toward a .lambda./2 plate by a reflecting means such as a mirror or the like. According to the former process, one unit of optical block is employed. According to the latter process, one or two units of optical block are symmetrically arranged.
FIG. 12 shows the structure of a conventional polarizing means and optical paths. A light source 40 comprises a halogen lamp, a metal halide lamp, or the like. Light emitted from the light source is passed through an optical block 50, which applies only a P wave to a liquid crystal panel (not shown). The optical block 50 comprises a plurality of prisms 50a-50f of glass which are bonded together. PBSs 52 are disposed between the prisms 50b, 50e and between the prisms 50c, 50d, and wave plates 53 are disposed on front faces of the prisms 50a, 50f. P+S waves emitted from the light source 40 are represented by solid arrows. The path of a P wave separated by the optical block 50 is represented by white blank arrows, and the path of an S wave separated by the optical block 50 is represented by hatched arrows.
P+S waves emitted from the light source 40 are separated by the PBSs 52. The P wave passes through the PBSs 52 and is applied to the liquid crystal panel. The S wave is reflected by the PBSs 52, then reflected forward by the prisms 50a, 50f, and converted by the wave plates 53 into a P wave, which is applied to the liquid crystal panel. Therefore, only the P wave is emitted from front faces of the prisms 50d, 50e and the wave plates 53. In this manner, either one of the P+S waves emitted from the light source 40 is applied by the optical block 50 to the non-illustrated liquid crystal panel.
If the optical block 50 were not employed, then the aperture of the light source 40 would be similar in shape to the effective area of the liquid crystal panel. However, a liquid crystal panel for displaying horizontally long images having an aspect ratio of 16:9, for example, has its side areas that cannot uniformly be irradiated with light, and hence cannot have uniform illuminance. Furthermore, since it is difficult for rays of light emitted from a lamp light source of a large divergent angle to be applied efficiently to a liquid crystal panel, it is known to use a multilens array composed of a number of small lenses, for example, as an optical means, to increase rays of light which reach a liquid crystal panel and uniformize a distribution of illuminance.
If such a multilens array is used, then, as shown in FIG. 13, the multilens array is of a shape similar to a liquid crystal panel as a light modulating means and having an aspect ratio equal to the aspect ratio of an effective aperture of the liquid crystal panel. The multilens array comprises a plane multilens array 54 composed of a matrix of convex lenses 54a and positioned closer to a light source (not shown) and a plano-convex multilens array 55 composed of a plurality of convex lenses 55a facing the convex lenses 54a of the plane multilens array 54. The multilens array applies rays of light from the light source efficiently and uniformly to the effective aperture of the liquid crystal panel.
Rays of light emitted from the light source of a liquid crystal projector and applied to the multilens array 54 are converged onto the convex lenses 55a of the multilens array 55 by the convex lenses 54a. The rays of light applied to the convex lenses 55a are applied to a condenser lens 56 by a convex lens 55b on the exit side of the multilens array 55. After the rays of light are modulated by a liquid crystal panel 57 associated with front and rear polarizers, they are applied to a cross dichroic prism 58. Before the rays of light are applied to the condenser lens 56 by the convex lens 55b, they are separated into R, G, B light by optical elements such as dichroic mirrors. The dichroic prism 58 comprises four prisms bonded together by reflecting surfaces 58a, 58b in the form of thin films having predetermined reflecting characteristics.
In FIG. 13, only a path of green light G is indicated by the solid lines. However, red light R and blue light B are similarly optically modulated by respective liquid crystal panels (not shown), and thereafter, as indicated by the arrows, applied from respective different directions to the cross dichroic prism 58.
The red light R modulated by the liquid crystal panel 57 is reflected by the reflecting surface 58a of the dichroic prism 58 toward a projection lens (not shown). The blue light B is reflected by the reflecting surface 58b of the dichroic prism 58 toward the projection lens. The green light G passes through the reflecting surfaces 58a, 58b. The red light R, the green light G, and the blue light B are combined with each other into light along one optical axis, generating a color video signal which is applied to the projection lens (not shown). As described above, the red light and the blue light are applied from the respective directions indicated by the arrows to the cross dichroic prism 58, and reflected by the respective reflecting surfaces 58a, 58b to the projection lens.
With the multilens arrays 54, 55 of matrices of convex lenses 54a, 55a, the rays of light emitted from the light source can be applied to the effective aperture of the liquid crystal panel 57 more efficiently and uniformly than if only the condenser lens 56 were employed.
In FIG. 14, the optical block 50 is positioned at the aperture of the light source 40, and the multilens arrays 54, 55 are positioned at the aperture of the optical block 50. This arrangement is effective to utilize rays of light emitted from the light source 40 more efficiently than the arrangements shown in FIGS. 12 and 13.
The conventional optical block 50 has an exit surface wider than an entrance surface thereof, and hence tends to increase the angle of incidence of rays of light on the liquid crystal panel, resulting in a reduction in the contrast. If the light source 40 is designed for a smaller size in order to prevent such a reduction in the contrast, then the optical block 50 has to utilize rays of light having a large divergent angle. Moreover, since the entrance side of the conventional optical block is of a size proportional to the aperture of the light source 40, the exit side of the optical block is of a size greater than the light source 40. Consequently, the optical block needs a considerably large installation space and is costly.
If only the multilens arrays 54, 55 are provided, then because the randomly polarized light emitted from the light source is applied directly to the polarizers, about 60% of the entire quantity of the light is blocked, and hence the efficiency with which the light source is utilized is poor. If the optical block 50 and the multilens arrays 54, 55 are combined with each other, then the multilens arrays 54, 55 are of an increased size commensurate with the exit aperture of the optical block 50, resulting in an increase in the length of the optical path from the multilens array 55 to the liquid crystal panel 57, so that the overall apparatus has a large size.
In order to solve the above problems, the applicant has proposed a light source for a display apparatus which comprises multilens arrays and an optical block having a plurality of joined prisms (see Japanese patent application No. 7-290570). FIG. 15 shows the proposed light source as incorporated in the optical system of a liquid crystal projector. As shown in FIG. 15, a light source 110 comprises a metal halide lamp 110a disposed at the focal point of a parabolic mirror for emitting light substantially parallel to the optical axis of the parabolic mirror from its aperture. An IR-UV blocking filter 111 blocks unwanted rays in infrared and ultraviolet ranges of the light emitted from the light source 110, and passes only effective rays of light to a next optical means.
The optical means has a first multilens array 112 comprising a matrix of convex lenses 112a and a second multilens array 113 comprising a matrix of convex lenses 113a, two of which face each of the convex lenses 112a of the first multilens array 112. An optical block 101, which will be described later on, is disposed between the first multilens array 132 and the second multilens array 113.
The optical block 101 comprises a plurality of bonded prisms. Rays of light converged by the first multilens array 112 are applied to a certain one of the prisms of the optical block 101. The optical block 101 converts randomly polarized light (P+S waves) into a P wave (or an S wave), which is applied to some of the convex lenses 113a of the second multilens array 113. The P or S wave is then separated by various optical elements into R light, G light, and B light, which are applied to liquid crystal panels. Therefore, the first multilens array 112, the optical block 101, and the second multilens array 113 allow rays of light emitted from the light source 110 and passing through the IR-UV blocking filter 111 to be applied efficiently and uniformly to the effective apertures of the liquid crystal panels.
Dichroic mirrors 114, 119 for separating rays of light from the light source 110 into red light, green light, and blue light are disposed between the optical block 101 and the effective apertures of the liquid crystal panels. In FIG. 15, the red light R reflected by the dichroic mirror 114 has its direction of travel bent 90.degree. by a mirror 115, and is applied through a condenser lens 116 to a red liquid crystal panel 117.
The green light G and the blue light B which have passed through the dichroic mirror 114 are separated from each other by the dichroic mirror 119. The green light G is reflected by the dichroic mirror 119 so that its direction of travel is bent 90.degree., and is applied through a condenser lens 120 to a green liquid crystal panel 121. The blue light B passes straight through the dichroic mirror 119, and travels through a relay lens 122, a mirror 124a, a relay lens 123, a mirror 124b, and a condenser lens 125 to a blue liquid crystal panel 126.
The red light, the green light, and the blue light which are optically modulated by the liquid crystal panels 117, 121, 126 are combined by a cross dichroic prism 118 as a light combining means. The red light R is reflected by a reflecting surface 118a toward a projection lens 130, and the blue light B is reflected by a reflecting surface 118b toward the projection lens 130. The green light G passes through the reflecting surfaces 118a, 118b. The R, G, B rays of light are thus combined to travel along one optical axis, and projected at an enlarged scale onto a screen (not shown) by the projection lens 130.
FIG. 16 is a perspective view of the optical block 101 as viewed from its front, and FIG. 17 is a fragmentary plan view of the optical block 101. As shown in FIGS. 16 and 17, the optical block 101 comprises triangular prisms 100a, 100e and parallelogrammatic prisms 200a, 200b, 200c, 200d, 100b, 100c, 100d which are bonded together. The randomly polarized light (P+S waves) emitted from the light source 110 and passing through the first multilens array 112 is applied to the optical block 101, as indicated by the solid arrows, and only a P wave is emitted from each of the prisms as indicated by the white blank arrows.
PBSs 103 (103a, 103b, 103c, 103c) for reflecting an S wave and passing a P wave therethrough, for example, are disposed on respective slanted exit ends of the prisms 200 (200a, 200b, 200c, 200d). The P wave that has passed through the PBSs 103 are emitted forward from front surfaces of the prisms 100 (100a, 10b, 100c, 100d). Mirrors 104 (104a, 104b, 104c, 104d) for reflecting forward the S wave reflected by the PBSs 103 are disposed on slanted surfaces of the prisms 200 which face the PBSs 103. 1/2 .lambda. plates 105 (105a, 105b, 105c, 105d) are disposed on front surfaces of the prisms 200 for rotating the plane of polarization of the S wave reflected by the PBSs 103 thereby to convert the S wave into a P wave, which is emitted forward.
Accordingly, the prisms 200 serve as an entrance region of the optical block 101, and rays of light applied to the prisms 200 are separated or polarized by the PBSs 103 and the 1/2 .lambda. plates 105, and a P wave is emitted forward from the prisms 100, 200. There are as many prisms 200 as the number of the convex lenses 112a of the first multilens array 112.
The optical block 101, which is composed of the prisms, the PBSs, the mirrors, etc., is capable of converting applied rays of randomly polarized light (P+S waves) into a P wave and emitting the P wave, and has entrance and exit sides whose areas are equal to each other. Since the optical block 101 is of a thinner structure than the conventional optical block, the optical block 101 is a space saver.
The optical block 101 is disposed between the first multilens array 112 and the second multilens array 113, as shown in FIG. 18, thereby providing the optical system as shown in FIG. 15.